The invention concerns a process for the production of a heat-treated, coarse fiber of polyester-POY, preferably in the form of a weft yarn for a tire cord fabric as well as a polyester fiber produced according to this process.
Serving as starting material for the process according to the invention is a polyester-POY, manufactured on the basis of an at least 90% polyethylene terephthalate, having a titer of 50-300 dtex and 20-50 fibrels.
Automobile tires of known structural type are composed in their essential parts of a stress carrier in the form of cords of synthetic yarns, which are embedded, in the form of a cord web, into rubber. The exterior appearance and, in particular, the running surface, are determined by the rubber.
In order for such tires with a cord web of synthetic filaments to have good running characteristics and a long useful lifetime, two essential conditions, among others, must be fulfilled: the tires must be very uniform in their shape, and their dimensional stability must remain as great as possible during use, even at increased velocity. It is a prerequisite for this that the cord fiber framework be spread and distributed as uniformly as possible during tire manufacture. In particular, the weft fiber per se must not therewith oppose disturbing resistance. In order to attain this condition, processes are known in which the carcass cords are parallelly arranged and calendared directly into the rubber, or where the web weft is intersected between the cord chain fibers before the calendaring. However, both processes are not easy to control insofar as reliability is concerned.
Other techniques are known, whereby the tire cord web is manufactured with slightly elastic weft.
Such weft yarns are composed either of polyester or nylon POY-filament yarns, which attain very high elongation values through extensive outshrinking. The initial strength required for working-up, as well as sufficient sliding strength and heat resistance can be attained by means of supplementary twisting around of cotton staple yarn. It is a peremptory disadvantage of this yarn, however, that it displays fluctuating initial strength, on account of the never entirely uniform twisting around of cotton along its fibers, which then later manifests itself negatively through a uniform spreading of framework cord in the tires.
Other tire cord webs are known, in which the weft fibers are composed of POY-polyester filaments, and which for fixing or for reduction of the shrinkage, are subjected to an additional heat treatment under determined tension on appropriate yarn carriers, and for prevention of slippage between the chain fibers are pre-treated with a rubber latex or are twisted around with cotton fibers (DE-A-27 48 747). The relaxation corresponds only to the portion possible by means of the winding. With the known weft yarns, the friction stability is effected by means of a latex mantle. The breaking elongation is determined to be from 80 to 250%.
The known processes display several disadvantages. The heat treatment is performed in conjunction with a determined fiber traction, in order to avoid excessive shrinkage. The forces resulting therewith differentially load the fiber inside the winding in its most sensitive circumstance, the result of which can be differences in shrink and tension.
With high temperature impregnation webs (i.e. approx. 240.degree. C.) using the known weft yarns of less than 100% residual breaking elongation, the wefts in the web frequently tear when the tire blank (unfinished tire) is brought into its required shape by means of expansion.
A further disadvantage is the employment of a rubber latex for prevention of slipping of the weft fibers along the longitudinal side of the chain fibers, whereby an additional process step is necessary, which disadvantageously influences the economy of the process.
Finally, it is a considerable disadvantage that the employment of latex produces latex powder (dust) during the weaving. The powder is composed of electrically charged rubber particles which are removable only with difficulty and expense, and which can lead to operational faults of the web.